U.S. Pat. No. 5,646,415 discloses an apparatus for inspecting radial skew and tangential skew on the basis of a spatial difference between nearby sensor outputs and a sensor output of a center address position.
Tilt inspection systems may determine tilt by measuring deflection angle of a beam of light reflected from an object as disclosed in the above-noted co-pending patent application.
Such systems may utilize a lateral effect photodiode position sensing detector or device (PSD) which is continuous and allows for greater accuracy than multicell detectors.
Preferably, it is desirable to use a position sensing detector which is small and which is relatively low cost. Such photodiodes are available from On-Trak Photonics, Inc. of Lake Forest, Calif. and have the designation 2L2SP or 2L4SP to indicate that the photodiode is a duo-lateral, two-dimensional position sensing detector.
One problem with use of such a relatively low cost position sensing detector is that slight alignment errors in the photodiode and/or its active area causes undesirable interaction between the X and Y axes of the active area. For example, the detector active area which comprises a thin rectangular slice of silicon may have a two degree rotational error with respect to the rest of the photodiode which results in an unacceptable amount of error in determining deflection angle and thereby tilt.
An object of the invention of the above-noted patent application is to provide a method and system for measuring a deflection angle of a beam of light reflected from a disk to detect tilt of the disk wherein alignment errors are corrected by processing reference data related to a known tilt of a reference disk with at least one electrical signal generated by a photodetector positioned in a detector plane to detect a focused spot of the reflected beam.
One problem of utilizing such a reference disk is that it may be difficult to manufacture such a reference disk that has the same value of tilt for both inner and outer radii. On systems with two or more tilt heads at differing radii, one must enter additional data pairs for each additional head up to a total of 30 entries for a 3 head tilt when using 5 known tilt angles.